The present invention relates generally to the field of manufacture of optical data storage disks, and in particular, to an optical disk mastering process for use in a disk molding process, capable of molding data storage disks containing a high density of information.
Optical disks are produced by making a master which has a desired surface relief pattern formed therein. The surface relief pattern is created using an exposure step (e.g., by laser recording) and a subsequent development step. The master is used to make a stamper, which in turn is used to stamp out replicas in the form of optical master substrates. As such, the surface relief pattern, information and precision of a single master can be transferred into many inexpensive replica optical disk substrates.
During the mastering exposure step, the mastering system synchronizes the translation position of a finely focused optical spot with the rotation of the master substrate to describe a generally concentric or spiral pattern of a desired track spacing or xe2x80x9ctrack pitchxe2x80x9d on the disk. The generally spiral track forming the desired surface relief pattern as a result of the mastering process can be defined by high regions termed xe2x80x9clandsxe2x80x9d and lower adjacent regions termed xe2x80x9cgroovesxe2x80x9d and/or pits (i.e., interrupted grooves). The recording power and size/shape of the focused optical spot (spot size) as well as the photosensitive material parameters determine the final geometry revealed in the master disk during the subsequent development step. Normal mastering practice uses high contrast positive photoresist for the photosensitive material.
Conventional mastering typically utilizes laser light with wavelength, xcex, in a range of 350 nm less than xcex less than 460 nm focused through an objective with a numerical aperture (NA) of 0.75 nm less than NA less than 0.90 to give a theoretical Gaussian spot size of:
SS=0.57xcex/NA (full width at half maximum intensity (FWHM)). Thus, a 350 nm laser light with NA=0.9 gives a theoretical spot size 0.22 microns (FWHM) as the practical limit for conventional optics.
After the master is recorded, it is flooded with developer solution to reveal the exposure pattern applied by the master recording system. The dissolution of the photoresist in the developer solution is in proportion to the optical exposure previously received in the recording process. The dissolution rate of the photoresist can be modeled for given exposure and development conditions (see Trefonus, P., Daniels, B., xe2x80x9cNew Principal For Imaging Enhancement In Single Layer Positive Photoresistxe2x80x9d, Proc. of SPIE vol. 771 p.194 (1987), see also Dill F. et al., xe2x80x9cCharacterization of Positive Photoresistsxe2x80x9d IEEE Transactions on Electronic Devices, vol. ED-22 p. 445 (1975).) Expressions explained in these referenced technical papers can be used to model the effects of exposures from several adjacent tracks recorded in the photoresist and subsequently developed. The photoresist dissolution in the developer solution is in proportion to the optical exposure previously received (positive type resist). More accurately, the dissolution rate (R) is given by the Trefonas model as
R[nm/sec]=R0xc3x97(1xe2x88x92M)q+Rb 
Where R0 and Rb are the dissolution rates of the fully exposed and unexposed photoresist (respectively), q is a resist parameter related to the resist contrast and M is the fractional unconverted photoactive compound in the resist. Typical values for commercially available resists are q=3, 10 less than R0 less than 200 [nm/sec] and Rb=0 for normal developer concentrations. The M term is dependent in a pointwise fashion on how much exposure was received in the resist (E(x,y,z)) and the resist""s parametric sensitivity xe2x80x9cCxe2x80x9d per the Dill convention:
M(x,y,z)=exp{xe2x88x92Cxc3x97E(x,y,z)}. 
Since optical disk mastering typically uses only 50-200 nm of photoresist thickness, the z-dependence of exposure can safely be ignored so that the above equations can be combined to give
R=R0(1xe2x88x92exp{xe2x88x92CE(x,y,)})q; 
or, with the exposure profile explicitly circular gaussian we may simplify to
R=R0(1xe2x88x92exp{xe2x88x92CkP exp [xe2x88x92r2/SS2]})q; 
Where r measures the radial distance from the center of the spot (r2=x2+y2), p is the recording power and k is a normalization constant for the guassian function. This dissolution rate, multiplied by the development time (td), gives the depth of photoresist lost from its initial coating thickness (T0), so that the final resist thickness (T(t)) is given by T(td)=T0xe2x88x92td R0(1xe2x88x92exp{xe2x88x92CkP exp [xe2x88x92r2/SS2]})q; From this expression one can see now optical exposure (P), development (td, R0) and photoresist thickness (T0) determines final surface relief pattern.
In some aspects, these expose/development processes may be compared with conventional photography. In photography, either exposure or development may be controlled/adjusted as necessary to obtain desired final development pattern. In this sense, one may consider the expose/development level as one process variable which may alternatively be controlled by recording power, development time, developer concentration, etc.
In the mastering process, it is desirable to simultaneously obtain wide lands (for user recorded features) and grooves of suitable depth for adequate tracking signals (e.g., greater than 50 nm). Higher density data storage disks often require the storage or a greater amount of information within the same or smaller size of disk area, resulting in smaller track pitch (i.e., distance between tracks) design criteria.
Attempts have been made to meet these design criteria. In prior art FIGS. 1-3, surface relief patterns of exemplary master disks formed using conventional disk mastering techniques are illustrated using the above expressions to model the effects of exposures from several adjacent tracks recorded in the photoresist layer and then developed. These comparisons assume (1) typical photoresist and developer parameters, (2) constant development time (=40 sec.), (3) SS=0.23 microns, (4) track pitch of 0.375 microns and initial photoresist thickness of 100 nm. As recording power (or alternatively, development time) is increased to obtain deeper grooves, the residual land width diminishes and lands become more rounded due to overlap exposure from adjacent tracks. Partially developed photosensitive material exhibits a granular roughness greater than that of the photosensitive material as initially coated on the disk. Roughness of lands worsens with deepening of grooves, resulting in additional noise in data readback.
More problems occur when the track pitch approaches the finite size of the mastering spot size. For formats where the desired track pitch is much larger ( greater than 2xc3x97) than the finite size of he mastering spot size (ss), the photosensitive material erosion of the lands is negligible and conventional mastering can provide wide lands with a  greater than 50 nm groove depth. However, for formats where the track pitch is  less than 2xc3x97 larger than the spot size, conventional mastering requires a compromise of either land width, groove depth, or both (due to overlap exposure from adjacent tracks).
In FIG. 4, exemplary embodiments of the mandatory link between land width and groove depth when using conventional mastering processes is illustrated. (Examples of 0.375 micron and 0.425 micron track pitch with 0.22 micron recording spot size). As the groove depth increases, the land width decreases. The master surface relief pattern geometries (i.e, land width/groove depth) are constrained for given conditions of track pitch and mastering spot size. This means the designer may not independently specify the desired parameters for replica land width and replica groove depth.
A secondary problem for conventional mastering is that the land width precision is limited by mechanical track pitch precision (e.g., mechanical precision of master recording system), which is increasingly difficult to control as track pitch decreases.
The present invention provides a data storage master disk and method of making a data storage master disk wherein the user may independently specify the parameters of replica land width and replica groove depth. The data storage master disk is for use in a data storage disk molding process for producing replica disks which are capable of storing a high capacity of information using a variety of disk formats.
In a first embodiment, the present invention provides a method of making a data storage master disk for use in a data storage disk molding process. The data storage disk molding process produces replica disks having a surface relief pattern with replica lands and replica grooves. The method includes the step of providing a master substrate. The master substrate is covered with a layer of photosenstive material having a specified thickness. A surface relief pattern having master lands and master grooves is recorded in the data storage master disk, including the steps of exposing and developing the photosensitive material. The exposing and developing of a specified thickness of a photosensitive material is controlled to form master grooves extending down to a substrate interface between the master substrate and the layer of photosensitive material, such that the width of the master grooves at the substrate interface corresponds to a desired width of the replica lands.
The thickness of the photosensitive material is specified and controlled to correspond to a desired depth of the replica grooves. In another aspect, the thickness of the photosensitive material is specified and controlled in dependence on master recording system spot size, desired track pitch, and desired depth of replica grooves. The step of controlling the exposure and development of the data storage master disk may include the step of controlling the exposing and developing of the photosensitive material to obtain a flat master groove bottom. In another aspect, the step of controlling the exposure and development of the data storage master disk includes the step of controlling the exposing and developing of the photosensitive material to obtain a smooth, flat master groove bottom, with smoothness determined by the master substrate.
The step of controlling the exposing and developing of the photosensitive material may include the step of controlling optical energy for exposing the photosensitive material to a degree sufficient to obtain a desired master groove bottom width after development and removal of the photosensitive material. In another aspect, the step of controlling the exposing and developing of the photosensitive material may include the step of controlling the development of the photosensitive material to a degree sufficient to obtain a desired master groove width after development and removal of the exposed photosensitive material.
The step of exposing and developing the data storage master disk may include the step of forming a groove bottom, wherein the groove bottom is flat relative to the master land. The step of exposing and developing the data storage master disk results in the data storage master disk having a master surface relief pattern defined by the master lands and the master grooves, wherein the surface relief pattern of the replica disks has an orientation which is inverse the orientation of the data storage master disk surface relief pattern.
The present invention may further provide the step of polishing the master substrate optically smooth; and forming a smooth master groove bottom using the master substrate. In one aspect, the step of providing a master substrate includes forming a master substrate made of glass. Preferably, the glass is polished. The photosensitive material may be bonded to the master substrate with or without intermediate layers.
The present invention may further provide for forming a first stamper using the data storage master disk. Replica disks are made using the first stamper. The step of making replica disks using the data storage master disk may be accomplished using a multiple generation stamper process.
In another embodiment, the present invention provides a method of making a replica disk from a master disk using an inverse stamping process. The replica disk is capable of storing high volumes of information. The replica disk includes a surface relief pattern with replica lands and replica grooves. The method includes the step of providing a master substrate. At least a portion of the master substrate is coated with a layer of photosensitive material to form the master disk. A surface relief pattern having master lands and master grooves is recorded in the master disk, including the steps of using a laser beam recorder for exposing the photosensitive material in a desired track pattern having a track pitch, and developing the photosensitive material. The exposing and developing of the photosensitive material is controlled for forming master grooves extending down to a substrate interface between the master substrate and the photosensitive material, such that the width of the master grooves at the substrate interface corresponds to a desired width of the replica lands. A first stamper is formed from the master disk. A second stamper is formed from the first stamper. A replica disk is formed from the second stamper, the replica disk including a surface relief pattern having an orientation which is the inverse of the master disk.
The present invention may further provide the step of controlling the thickness of the layer of the photosensitive material to correspond to a desired depth of the replica grooves. The specified and controlled thickness of the photosensitive material depends on master recording system spot size, desired track pitch, and desired depth of replica grooves.
The step of controlling the exposing and developing of the photosensitive material may include the step of controlling the exposing and developing of the photosensitive material to obtain a flat master groove bottom. Recording a desired track pitch in the photosensitive material may further include the use of a focused laser beam at a spot size which is greater than one half of the track pitch.
The step of a master substrate may include providing a master substrate made of glass. Further, the master substrate may be polished.
In one aspect, the desired track pattern is a spiral track defined by adjacent master lands and master grooves, wherein the steps of exposing/developing the master disks includes forming a wide, flat master groove bottom defined by the disk substrate. The step of recording the master disk includes forming master groove bottoms having a width which does not necessarily depend on the depth of the master groove for a desired track pitch. The resulting depth of the master groove is dependent on the specified thickness of the photosensitive material and the cumulative optical exposure received by the photosensitive layer at a position half way between two adjacent tracks. In particular, this depends on the desired groove bottom width and the ratio of master recording spot size to desired track pitch.
In another embodiment, the present invention provides a master disk. The master disk includes a master substrate. A layer of photosensitive material covers at least a portion of the master substrate. The photosensitive material includes a surface relief pattern in the form of a track pattern defined by adjacent master lands and master grooves. The master grooves extend down to the disk substrate, the master grooves including a master groove bottom and the master lands including a master land top, wherein the master groove bottom is wider than the master land top.
The master groove bottom is generally flat. In particular, the master groove bottom is flat relative to the master land top, and in particular, the master groove bottoms may be wide and flat relative to the master land tops. Preferably, the master groove bottoms include sharp corners. Additionally, all of the master groove bottoms on the exposed/developed master disk are level with each other to the precision of the master substrate flatness. This is important in flying head media applications, such as near field recording techniques, where small lenses fly in proximity to the replica disk surface.
The master grooves may include a groove depth which is proximate the thickness of the photosensitive material for cases where the track pitch is greater than approximately 1.6 times the spot size. In one aspect, the master grooves include a groove depth which is greater than 50 nanometers, track pitch is less than two times the mastering system spot size, and the width of the master groove bottom is greater than 25 percent of desired track pitch. In another aspect, the width of the master groove bottom is greater than 50 percent desired track pitch.
In another embodiment, the present invention provides a disk including a replica substrate having a first major surface and a second surface. The first major surface includes a surface relief pattern in the form of a track pattern defined by adjacent lands and grooves. The track pattern having a track pitch less 0.425 nanometers, wherein the grooves extend down into the disk substrate. The grooves include a groove bottom and the replica lands include a land top, wherein the land top is flat. This is particularly important in near field recording techniques, wherein lens-to-media-surface separation is extremely critical.
In one aspect, the land top has a width greater than 25 percent of track pitch. In one preferred aspect for the track pitch less than or equal to 400 nanometers, the groove depth is greater than 80 nanometers and the land width is greater than 160 nanometers. Preferably, the land top is smooth and has sharp edges. In one preferred embodiment, the land tops are level with each other to the precision of the flatness of the master disk substrate. The land tops are level and at the same elevation relative to the second major surface. This is important in flying head media applications, such as near field recording techniques, where small lenses fly in proximity to the replica disk surface.